Updated documentation to indicate es.wait may return a buffer

[samolsen]

es.wait was changed to support Buffer concatenation, and may not always pass a string to its second callback argument. This updates the documentation to indicate the type may be a String or Buffer.

[dominictarr]

thanks. Merged into 3.2.1

I also want to say, this is an explar pull request. even though it's just a simple documentation fix you explained the context i needed and that made merging a breeze!